The fMRI scanner tracks the change in blood flow to the amygdala. "We are looking at signal changes in particular parts of the brain," Hirsch says, "The signal means increased neural activity." The magnetic resonance signal is responding to the amount of blood that is being recruited to the local area. The photograph of a fearful face elicits a greater amount of blood flow and a higher signal during the scanning period than a neutral face photograph."
Critics of fMRI-based research point out that it is not always clear what the flow of blood in a brain region means. But Hirsch dismisses naysayers. "We use very carefully chosen stimuli that don't necessarily scare people [who are] in the scanner, but arouse systems that are involved in fear activity if you were scared," she says. "The interpretation of another fearful face arouses the system of neural regions that respond to fear in the observer."
Hirsch notes that the amygdala responds to more than just facial expressions. "If you were in a dark alley and something scary jumped out at you," she says, "it would be the amygdala that would contribute to your decision to run."
Fear is as basic a human process as breathing or digestion, yet science's ability to completely understand and describe it remains elusive. "That is the $64,000 question," Lewis says, noting that despite more than 100 studies into how the body reacts to fear, there still is no way to quantify fear itself.